The use of offshore mobile units for drilling and producing operations has become relatively common place in recent years. As the supply of oil becomes more scarce, exploration and production activities are being pushed into more environmentally hostile areas. For example, in colder regions, ice masses, driven by wind and currents, pose particular problems. Although the smaller ice masses can be diverted by various methods, the larger ones cannot be so handled and the threat of collision requires the operation to be shut down and the vessel relocated as quickly as possible to void injury to personnel and property. Therefore, it is occasionally necessary to remove the offshore mobile unit and marine riser to avoid damage resulting from ice or severe storms. However, the installation and removal of an offshore unit and marine riser is a costly and time-consuming operation. A reduction in expense, time or effort provides more efficient exploration and production.
In drilling wells and producing oil at an offshore location, it is necessary that an elongated conduit, or marine riser, extend from a vessel through the water column to a primary borehole or wellhead. The riser encloses the drill string and permits fluid circulation during drilling operations. Risers are also used, after completion of the well, to transport hydrocarbons to the surface. Normally the risers are comprised of a series of 30 to 40 foot sections which are sealably joined to provide a single elongated conduit. These sections are stored horizontally on the deck of the ship and, when needed, are raised to a vertical position to be threaded onto the section below. A drill string is stored and assembled in the same fashion, but is assembled and lowered independently of the riser once it is in place. The riser is, ideally, assembled and disassembled once, while the drill string is assembled and disassembled many times (to change drill bits, insert tools, etc.).
Technology used heretofore employs either mechanical or remote methods (such as guideline, acoustic and/or television systems) to aid in the alignment of a marine riser and subsea wellhead when the riser is connected to the wellhead. For an example of wellhead connection equipment see Watkins, U.S. Pat. No. 3,265,130; Franks, U.S. Pat. No. 4,214,842; and British Patent Specification No. 1,341,047. If the vessel must be relocated from the drilling or producing site, the drill string and marine riser will normally be withdrawn sequentially by sections and stored on the vessel. The desirability of minimizing such a time-consuming and laborious operation is recognized in the art. For instance in U.S. Pat. No. 4,234,047, issued Nov. 18, 1980 to Geroge E. Mott, the use of a riser in two detachably connectable segments is advocated. If the vessel must be moved, the riser is disconnected at an intermediate point several hundred feet below the water surface. The upper riser segment is then withdrawn and the lower riser segment is left in place. Such a system is only practical, however, in waters of sufficient depth so that the lower remaining segment is not endangered. Another example of an attempt to minimize this riser withdrawal time is a riser set aside system as shown by Ilfrey et al., U.S. Pat. No. 4,147,221, in which the riser is not removed from the water column, but set aside onto an adjacent support. This system, however, still leaves the riser vulnerable to environmental hazards even more so than that described by Mott. Other riser systems are generally shown by: Watkins, U.S. Pat. No. 4,403,658; Buresi et al., U.S. Pat. No. 4,194,568; Walker, U.S. Pat. No. 4,126,183; and McCulloch, U.S. Pat. No. 3,664,437.
The present invention provides a method and means for storing and protecting a marine riser in seas where conditions may require moving the drilling vessel off its location and which may advantageously be used in both shallow and deep waters.